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Chapter 4: Electromagnetic Actuator Basics Chapter 4: Electromagnetic

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Chapter 4: Electromagnetic Actuator Basics Chapter 4: Electromagnetic Chapter 4: ElectroMagnetic Actuator Basics ChapterChapter 4:4: ElectroMagneticElectroMagnetic ActuatorActuator BasicsBasics Objectives The objectives for this chapter are as follows: • Discover what the components of an electromagnetic actuator are, how they are constructed and how the components interact. • Learn about the pancake and tubular shaped armatures as well as the three variations on the tubular shape. • Understand the concept of Force vs Air Gap and when to use the flat face, conical and proportional style actuators. • Learn what parameters affect Solenoid force and how the process of solenoid design must juggle these parameters depending on the application. Introduction In the previous chapters we learned about the basics of magnetics and electronics. In this chapter we will use these basics to describe the construction and operation of an electro-magnetic actuator. Various designs of actuators will be compared, as well as a description of how the performance of each varies. Page 39 Chapter 4: ElectroMagnetic Actuator Basics Electromagnetic Actuator Recall the definition of an electromagnetic actuator from Chapter 1. A solenoid valve is one which uses an electromagnetic actuator to move a hydraulic control element such as a poppet or spool. An electromagnetic actuator takes electricity and converts it into magnetic force. Magnetic force is used to move the spool or poppet which in turn controls the direction of flow. The actuator portion of the solenoid valve is highlighted in the following diagram. Electromagnetic Actuator Yoke (Frame or Shell) Air Gap Armature (Plunger) Coil Winding Guide Tube Pole Piece Termination Push Pin Page 40 Chapter 4: ElectroMagnetic Actuator Basics Actuator Components Coil winding The coil winding is the solenoid that we learned about in the previous chapter. As we learned, it creates an electromagnetic field when the current is applied across the terminals. It is typically made from copper which is the most cost effective and efficient conductor of electricity. Yoke The yoke, also referred to as the shell or frame, concentrates the magnetic field. It surrounds the outside of the solenoid coil and is typically made from low carbon steel. Recall from chapter three that steel or iron is a Ferromagnetic material. The flux created by the solenoid can flow easily through this Ferromagnetic material. A Ferromagnetic material can also intensify the magnetic field. If the yoke did not exist, the flux lines or magnetic field would be loosely spaced, and the actuator would be inefficient. Low carbon steel is used for most electromagnetic actuators. It has a high permeability (good conductor of magnetics), and a relatively low cost. There are better magnetic materials, but the increase in efficiency does not justify the increase in cost. The components of the actuator which are made from iron are, the yoke, pole piece and the armature. Yoke Yolk not Guide Tube The guide tube acts as a guide for the armature. It is typically made from a nonmagnetic material such a stainless steel. The material needs to be nonmagnetic to avoid the armature being attracted to it. Page 41 Chapter 4: ElectroMagnetic Actuator Basics Pole Piece The pole piece acts as a magnet when the current is applied to the winding. It is a fixed Ferromagnetic part of the armature. Refer back to the summary diagram in Chapter 3, in which two pieces of iron are shown. When the coil is energized, a magnetic field forms a north and south pole in these pieces of iron. The pieces are attracted and move towards one another. Similarly, in the pole piece, as the coil is energized, the pole piece and corresponding armature are attracted to one another. The pole piece however, is fixed into place and draws the armature toward it. Armature (Plunger) As mentioned in the pole piece description, the armature is the piece of iron which the pole piece attracts. This part is allowed to move freely, constrained only by the guide tube. If nothing opposes the movement of the armature, it will be attracted to, and move towards the pole piece. However, if something does oppose the movement of the armature, it will exert a force on that object. Air Gap The air gap is the distance between the pole piece and the armature. The size of the air gap depends on the product which the electromagnetic actuator is coupled (hooked up) to, such as the stroke of the spool or pilot pin. If the actuator was not connected to anything, the air gap would not exist, because the parts would remain attracted to each other once the current was applied. There would be no force to break the magnetic attraction between the pole piece and the armature. Push Pin (Connecting Element) The push pin is the element that transfers the magnetic force to the part outside of the actuator. In addition, any forces which oppose the magnetic force will be transferred to the armature through the push pin. The push pin is typically made of a nonmagnetic material such as stainless steel so that it is not attracted to the pole piece. Page 42 Chapter 4: ElectroMagnetic Actuator Basics Armature Shapes There are two basic shapes of armatures; pancake and tubular. The tubular style has three variations; flat, conical and proportional. The following sections describe each. Pancake The pancake style armature is used in applications where a high holding force is required but the air gap initially separating the plunger and pole piece is small. The name pancake comes from the shape of the flat armature which is the same diameter as the outside of the yoke. Holding force refers to the magnetic force between the pole piece and armature, when the air gap is zero. The following diagram shows a cross section of a pancake style actuator. Winding Armature Pole Piece Guide Tube Yoke Guide Tube The graph below shows force vs. the air gap. This represents the magnetic attractive force between the pole piece and the armature when a constant current is applied and the position of the armature changes. Notice that the force is high when the air gap is zero (pole piece and armature are in contact), but decreases sharply as the air gap is increased. Force 0 Air Gap Page 43 Chapter 4: ElectroMagnetic Actuator Basics Tubular The tubular shape armature is used in hydraulic valves and pneumatic valves. There are three variations on the tubular shape, based on the shape of the pole piece and armature; flat, conical, and proportional. A diagram of a tubular shaped actuator is shown below. Termination Wire Pole Piece Push Pin Guide Tube Coil Winding Yoke (Frame or Shell) Armature (Plunger) Air Gap As with the pancake shape actuator we can plot a force vs air gap graph (see the following graph). The tubular shaped armature gives flexibility in controlling the shape of this graph. Three basic types of armature and pole piece designs are outlined and described in the following sections. Refer to the following comparison graph while reading through the descriptions of each shape. The flat face armature (number one on the graph) has a high holding force (magnetic force between the pole piece and armature, when the air gap is zero). This style of actuator is characterized by a low force at the full open point and a high force when the air gap is zero. The full open point is the initial position of the product which the armature is connected to. This point varies depending on the application of the solenoid. It is defined by the distance the armature will be required to move Page 44 Chapter 4: ElectroMagnetic Actuator Basics The conical face armature (number two on the graph) has a high initial force with a steady increase. The curve for this actuator shows that the initial force at full open is higher than the flat face, but the holding force is lower. The proportional face armature (number three on the graph) has a constant force. The graph shows a constant or level force for the majority of movement (change in the air gap) of the armature. The application (what the solenoid will be hooked up to) usually dictates which force versus air gap curve will be used. Not only is the performance of the solenoid or actuator force considered, the cost is as well. Each style discussed requires a better manufacturing process with the flat face being the least costly and the proportional being most costly. 1 Flat Face (High Holding Force) Force Conical Face Proportional Face 2 (High Initial Force (Constant Force) Steady Increase) 3 0 Air Gap Full Open Page 45 Chapter 4: ElectroMagnetic Actuator Basics Flat Face The illustration and graph below show a flat face plunger and pole piece. Magnetic field lines are overlaid on top of these parts. These indicate that magnetic field lines jump directly across from the plunger to the pole piece. The tendency of the field lines is to exit through the armature face and enter through the pole piece face at a right angle or perpendicular to the surface. Recall from the previous chapter that the flux, or strength of the magnetic attraction between the parts is based on the magneto-motive force (nI), multiplied by the area through which the field flows (in this case the circular area of the armature and pole piece). The graph to the right represents the same actuator with various current levels applied. Each curve represents a different current value or a different value of am- pere-turns (magneto-motive force). The lowest curve represents the lowest current level. The shape of the curve is based on the shape of the pole piece, armature (the area which the flux passes through), and the amount the domains within the material are aligned (the amount of nI).
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